Changes in NOR activity pattern in the presence of supernumerary heterochromatin in the grasshopper Eyprepocnemis plorans

HP1 knockdown is associated with abnormal condensation of almost all chromatin types in a grasshopper (Eyprepocnemis plorans)

Heterochromatin protein 1 (HP1) is a highly conserved family of eukaryotic proteins required for heterochromatic gene silencing and euchromatic gene transcription regulation. In addition, HP1 is involved in chromatin organization and protection of chromosome integrity during cell division. Here, we present a cytological and molecular analysis of the effects of HP1 knockdown in Eyprepocnemis plorans, a grasshopper species polymorphic for supernumerary heterochromatic chromosomes. Our results revealed contrasting effects of HP1 knockdown on gene activity. While the Bub1 gene decreased in expression level in HP1 knockdown animals, NOR activity, rRNA and, contrarily to previous reports in Drosophila, Hsp70 gene expression remained unchanged. Furthermore, HP1 knockdown resulted in abnormal chromatin condensation, chromosomal bridges, higher frequency of macrospermatids, loss of muscle mass and hemolymph amount as well as a low number of dividing cells and survival reduction. All these phenotypes are very likely due to the chromatin condensation disruption observed for almost all kinds of chromatin.

Ribosomal DNA is active in different B chromosome variants of the grasshopper Eyprepocnemis plorans

B chromosomes are considered to be genetically inert elements. However, some of them are able to show nucleolus organizer region (NOR) activity, as detected by both cytological and molecular means. The grasshopper Eyprepocnemis plorans shows a B chromosome polymorphism characterized by the existence of many B variants. One of them, B24, shows NOR activity in about half of B-carrying males in the Torrox population. Molecular data have suggested the recent origin for B chromosomes in this species, and on this basis it would be expected that NOR activity was widespread among the different B variants. Here we test this hypothesis in four different B chromosome variants (B1, B2, B5, and B24) from 11 natural populations of the grasshopper E. plorans covering the south and east of the Iberian Peninsula plus the Balearic Islands. We used two different approaches: (1) the cytological observation of nucleoli attached to the distal region of the B chromosome (where the rDNA is located), and (2) the molecular detection of the rDNA transcripts carrying an adenine insertion characteristic of B chromosome ITS2 sequences. The results showed NOR expression not only for B24 but also for the B1 and B2 variants. However, the level of B-NOR expression in these latter variants, measured by the proportion of cells showing nucleoli attached to the B chromosomes, was much lower than that previously reported for B24. This suggests the possibility that structural or genetic background conditions are enhancing the expressivity of the rDNA in the B24 variant.

B-chromosome ribosomal DNA is functional in the grasshopper Eyprepocnemis plorans

B-chromosomes are frequently argued to be genetically inert elements, but activity for some particular genes has been reported, especially for ribosomal RNA (rRNA) genes whose expression can easily be detected at the cytological level by the visualization of their phenotypic expression, i.e., the nucleolus. The B(24) chromosome in the grasshopper Eyprepocnemis plorans frequently shows a nucleolus attached to it during meiotic prophase I. Here we show the presence of rRNA transcripts that unequivocally came from the B(24) chromosome. To detect these transcripts, we designed primers specifically anchoring at the ITS-2 region, so that the reverse primer was complementary to the B chromosome DNA sequence including a differential adenine insertion being absent in the ITS2 of A chromosomes. PCR analysis carried out on genomic DNA showed amplification in B-carrying males but not in B-lacking ones. PCR analyses performed on complementary DNA showed amplification in about half of B-carrying males. Joint cytological and molecular analysis performed on 34 B-carrying males showed a close correspondence between the presence of B-specific transcripts and of nucleoli attached to the B chromosome. In addition, the molecular analysis revealed activity of the B chromosome rDNA in 10 out of the 13 B-carrying females analysed. Our results suggest that the nucleoli attached to B chromosomes are actively formed by expression of the rDNA carried by them, and not by recruitment of nucleolar materials formed in A chromosome nucleolar organizing regions. Therefore, B-chromosome rDNA in E. plorans is functional since it is actively transcribed to form the nucleolus attached to the B chromosome. This demonstrates that some heterochromatic B chromosomes can harbour functional genes.

Quantitative analysis of NOR expression in a B chromosome of the grasshopper Eyprepocnemis plorans

The B24 chromosome in the Torrox population of the grasshopper Eyprepocnemis plorans is recurrently attached to a nucleolus in diplotene cells, indicating the activity of its distally located ribosomal DNA (rDNA). The frequency of males expressing the B chromosome nucleolus organizer region (B-NOR) almost doubled in 4 years. The likelihood of expressing the B-NOR increased with the B number and, in males expressing it, about 20% of their cells showed a nucleolus attached to the B. When active, the B-NOR contributed more than 25% of total cell nucleolar area (NA). Within males expressing the B-NOR, total cell NA did not differ between cells showing the active or inactive B-NOR, suggesting that total cell NA is tightly regulated in this species. However, this parameter tended to increase in this population from 1999 to 2004, in parallel to the neutralization process which is taking place in this population. Finally, an analysis of A chromosome NOR interdependence for activity revealed a positive correlation among autosomes but a negative correlation between autosomes and the X chromosome, the manifestation of which depends on B-NOR activity. These results are discussed in the context of the nucleolus as a sensor of the stress caused by parasitic B chromosomes.

Location and expression of ribosomal RNA genes in grasshoppers: abundance of silent and cryptic loci

We investigate regularities and restrictions in chromosome location of ribosomal RNA genes, analysed by fluorescent in situ hybridization (FISH), and their phenotypic expression assessed by nucleolus formation at first meiotic prophase cells, analysed by silver impregnation, in 49 grasshopper species. High variation was found for rDNA location between species within most genera analysed. The mean haploid number of rDNA loci detected by FISH was 2.47, but some species had up to 10 loci. Chromosome distribution of rDNA loci differed between the Gomphocerinae and Oedipodinae subfamilies, most loci being proximal to the centromere in the former and distal to it in the latter. Chromosomes 2, 3 and X frequently carried rDNA in Gomphocerinae species with 2n male symbol=17 chromosomes, whereas chromosomes 6 and 9 were the most frequent rDNA locations in the Oedipodinae. About 13% of the 126 rDNA loci detected by FISH were silent, although this figure might be even higher. The comparison of FISH and silver-impregnation results also suggested the existence of cryptic NORs, i.e. those forming small nucleoli with no apparent presence of rDNA revealed by FISH. This was especially clear after the same cells in two species were sequentially treated with both silver impregnation and FISH. The abundance of silent and cryptic loci might thus suggest that rDNA spreads through grasshopper genomes by the Dubcovsky and Dvorak mechanism-that is, the transposition of a few rRNA genes to new chromosome locations, their amplification giving rise to new NORs, and the elimination of the old NORs. The cryptic NORs might correspond to nascent NORs, i.e. a few rRNA gene copies moved to new locations, whereas the inactive rDNA loci might correspond to those being in the process of elimination.

Nucleolus size variation during meiosis and NOR activity of a B chromosome in the grasshopper Eyprepocnemis plorans

The number of nucleoli and nucleolar area were measured in meiotic cells from males of the grasshopper Eyprepocnemis plorans collected in three natural populations. Number of nucleoli per cell showed no significant correlation among cells in different meiotic stages, but there was strong positive correlation for nucleolar area between leptotene and interkinesis cells in individuals from distant populations (Salobreña in Spain, and Smir in Morocco). No correlation was, however, observed for both parameters between the meiotic stages analysed in individuals from the population of Torrox (Spain). The number of nucleoli at leptotene was about double the number at interkinesis, as expected from the double ploidy level at leptotene and the corresponding double number of rDNA clusters. Leptotene nucleolar area, however, was about fourfold that in interkinesis, presumably due to higher requirements for ribosome biogenesis in meiosis I than meiosis II. In Torrox, diplotene cells showed a lower number of nucleoli but larger nucleolar area than in leptotene cells, suggesting an increase in nucleolus size during prophase I. Significant differences were found among populations for nucleolar area but not for number of nucleoli, the smallest nucleolar area being observed in Torrox, which is the population harbouring the most parasitic B chromosome variant. No clear effects on nucleolar area or number of nucleoli were associated with the B-chromosome number. However, B-chromosome effects on the nucleolar area were apparent in the Torrox population when data were analysed with respect to a B-chromosome odd-even pattern in leptotene and interkinesis cells. However, in diplotene cells no odd-even pattern was observed for both nucleolar parameters, suggesting that the increase in nucleolar size from leptotene to diplotene dilutes the leptotene odd-even pattern. The rDNA distally located in the B chromosome was associated with a nucleolus in 6.5% out of the 247 diplotene cells analysed. The implications of these findings are discussed in the context of B chromosomes as stress-causing genome parasites and the nucleolus as a sensor of stress.

Comparative analysis of rDNA location in five Neotropical gomphocerine grasshopper species

We report here, for the first time, the chromosome complement, number and location of the nucleolar organizer regions (NORs) revealed by silver staining (AgNO(3)) and fluorescent in situ hybridization (FISH) in five Neotropical gomphocerine species: Rhammatocerus brasiliensis, R. brunneri, R. palustris, R. pictus and Amblytropidia sp. The objective of this study was to summarize available data and propose a model of chromosome evolution in Neotropical gomphocerines. All five species studied showed chromosome numbers consisting of 2n = 23,X0 in males and 2n = 24,XX in females. Amblytropidia sp. was the only species showing a bivalent (M(8)) with megameric behavior during meiosis. The rDNA sites were restricted to autosomal pairs, i.e. the pericentromeric region of the S(9) chromosome, the consensus NOR location in all five species. R. brasiliensis was the only species showing additional NORs on M(4) and M(6) pairs which, likewise the S(9) NOR, were active in all cells analyzed. Comparison of these results with those reported previously in Palearctic gomphocerine species suggests higher resemblance of Neotropical species with the Old World species also possessing 23/24 chromosomes. Evolutionary mechanisms responsible for the observed interspecific variation in NOR location in this group are discussed.

Comparative FISH analysis in five species of Eyprepocnemidine grasshoppers

The chromosomal localization of ribosomal DNA, and a 180 bp satellite DNA isolated from Spanish Eyprepocnemis plorans specimens, has been analysed in five Eyprepocnemidinae species collected in Russia and Central Asia. Caucasian E. plorans individuals carried each of the two DNAs, but the rDNA was limited to only two chromosomes (S(9) and S(11)) in sharp contrast to Spanish specimens that show 4-8 rDNA clusters and to Moroccan specimens which carry rDNA in almost all chromosomes. The four remaining species, however, lacked the 180 bp tandem repeat, and showed rDNA clusters in one (S(9) in Thisoicetrinus pterostichus), two (S(9) and S(10) in Eyprepocnemis unicolor; M(8) and S(11) in Heteracris adspersa), or three (S(9), S(10), and S(11) in Shirakiacris shirakii) chromosome pairs. The implications of these findings for the evolution of these two chromosome markers in this group of species are discussed.

Population differences in the expression of nucleolus organizer regions in the grasshopper Eyprepocnemis plorans

Fluorescence in situ hybridization revealed the presence of ribosomal RNA genes in paracentromeric regions of all A chromosomes and in the distal half of B chromosomes in embryonic cells from Moroccan specimens of the grasshopper Eyprepocnemis plorans. The expression of these genes was monitored by the presence of nucleoli attached to each chromosome bivalent in diplotene cells from males collected from two different Moroccan populations and was compared to previous data of Spanish populations. Whereas only the nucleolus organizer regions (NORs) on S9-S11 and X chromosomes were active in the Spanish specimens, Moroccan individuals showed NOR activity in all chromosomes. The rRNA genes on the B chromosome were inactive in both populations. The S9 and S10 NORs were less active in Moroccan specimens than in Spanish specimen, which might be partly explained by the negative inter-dependence for expression of the S10 NOR with respect to those on L2 and X chromosomes. On the other hand, the X NOR was more active in Moroccan specimens than in Spanish specimens, and this might be partly due to the positive effect that the presence of B chromosomes has on the expression of this NOR. The implications of these observations on current models of NOR activity regulation are discussed.

Unusually high amount of inactive ribosomal DNA in the grasshopper Stauroderus scalaris

Fluorescence in-situ hybridization (FISH) was employed to determine the chromosomal location of the ribosomal DNA cistrons in spermatocytes of two populations of the grasshopper Stauroderus scalaris. The results showed that paracentromeric C-bands, which in this species constitute about 50% of the total chromatin, contain substantial amounts of rDNA in all chromosomes. However, silver impregnation showed the presence of a single active nucleolus organizing region (NOR) in chromosome 3 of primary spermatocytes, indicating an extremely high amount of silent rDNA across the whole genome of this species in the two geographically distant populations analysed. The significance of such an unusual phenomenon is discussed.